This is really neat - they are installing a large solar photovoltaic array on the capped landfill, here in Maynard, MA. When it is up and running and connected, it will have a capacity of ~1MW, which is enough to power about 200 homes! It is being installed at zero cost to the town, and we will be paying just 2.5¢ per kWh for the first 10 years, and for 11-20 years, just 7.5¢ per kWh.

I was not remembering the output correctly - it is less than I thought it is. The total capacity is ~1MW and the site is 14 acres. According to the article, the output is only enough for part (~25%) of the town's buildings. That doesn't sound right though - it should be enough for about 200 homes. If the town buildings are using as much electricity as 800 homes, I'd like to know why it is that much.

I also think that the article badly misstates the total annual output. A 1MW system can put out as much as 12MWh per day, so that would total ~4.3GWh per year. (Maybe my math is off?)

Yeah, the math is odd with the little bit of info in the article. There's probably a big difference between a panel's max output and what's actually feasible due to placement/direction/latitude/air quality/altitude/weather/how much dust has accumulated/etc. The $0.025/kWh seems low.

Yes, for solar the power factor (the percent of power generated from the maximum) is around 30%. For a 1MW array, you'd expect peaks in the middle of the day (assuming it's sunny) that might get close to the max, but averaged over the day it should generate 7.2MWh instead of the max 24MWh. This array doesn't appear to have any tracking, and at the high latitude, it might get less than that. There might be some snow accumulation at night, but they may have heaters on them for that case. Now one thing that most people don't know is that solar panels are actually more efficient at colder temperatures, so that may help a little bit in winter, but since the sun is lower at that time of the year, it may not compensate. The odd thing about the article is that it doesn't mention power production in MWh, just MW. You would use megawatts for instantaneous power, but you need to put in a time factor (megawatt hours) to really know how much power is being produced in total. It's a fairly common problem in the press.

Panels do usually need some cleaning, if it's anything like my installation. I'm going to have to get up on the roof this weekend and do some cleaning, they got really dusty since the spring pollen hit.

Peak Sun = 9am to 3pm (During Winter) is used as the basis for total output of a panel. EG 200w Panel produces 1Kwh over this peak sun period. So if they're talking a 1Mwh system - that's the design rating. They'll easily hit this and may get more but the EE's designing this system always allow a fudge factor and for large PV Arrays that's about 25 percent according to the engineers on the two systems near me. This allows for line losses, panels that aren't tracking due to tracker motor failures and such so the systems are actually larger then rated.

From the little information that Neil provided, I suspect they're talking the expected output is 1Mwh for the day. Makes the Array a 200Kwh design (1000 x 200w per panel = 200,000 (200K) x 5 (peak sun) = 1Mwh). Keep in mind that I'm using output figures that are easy to work with for clarity. Neil can easily check the projected/completed number of panels in the array to see what they did and compare it against my figures. Hell go ask the guys, which is what I did. Most of them are pretty nice and quite willing to take a moment to answer a question or two during the construction stage.

You may be right, but I think the maximum capacity is 1MW, so the winter output would be 6-8MWh. The system on the parking lot at REI is 210kW maximum output, so the winter daily output would be 1.2-1.6MWh.

Okay, I've got a data point - an architect I know who lives in New Jersey has a 9.9kW solar PV system (44 225W Sunpower panels) and in 40 months of operation, he has averaged 750kWh/month, and that averages 75.75kWh per kW of maximum capacity per month.

So, the 1MW system (which is using 240W panels so that means ~4,167 panels in total) would have about 75MWh output per month by this estimate, or about 900MWh per year. So, the number in the 2010 Beacon Villager newspaper was probably correct - except for the units.

The article said 1,125 MW but they had to have meant 1,125 MWh or 1.125GWh.

750kWh/mo divided by 30.4 days/mo = 24.66kWh/day. Divide that by 9.9kW capacity and you get 2.49 hrs at peak output. With the average of 12 daylight hours over the year, the efficiency is 20.8% of peak. Interesting.

Keep in mind that my example was for a Single 1,000,000 watts output. This is the rated output - think lightbulb at 40w (multiply that figure by how many hours of use - that's your watt hours). Standard PV Design (AFAIK) always uses the peak sun figure to calculate the watt hour output of a system. This peak sun is the period from 9am to 3pm or 5 hours.

Now this doesn't mean the system wont put out more then that but these figures are like the MPG Estimates on new cars. Provide a comparison point that can be checked by the buyer. In any major engineering project, before it's approved, the engineers have to provide their figures and basis of calculation (prove their work) so it can be reviewed and it's the same in this case. As to how much actual power the system will provide, if you go by the engineers estimate, you'll have a good, conservative number to work from and just like with the MPG estimates, if you get more, you're quite happy with the outcome.

As you stated about the architect, he avgs. quite a bit per day from his system but I bet his is a grid-tie setup. This means that instead of batteries, he draws from the grid during the night or when his demand exceeds his output. Really nice if you're wanting to make a bit of money.

In our case, we're planning an off-grid system with the budget based on the cost to install a powerline ($20,000). So far, we've managed to drop our budget to half that ($10,000) meaning we'll see an immediate payback for the system. What we've changed from the base system cost of $5000 (1Kw output) was tripled our battery pack and chose a 230v 50a inverter. Our entire house is electric though we do use solar water heating for hot water and to heat the house during the winter. We're using hot water baseboard heaters and using ceiling fans to even temps out. Old designs but still usable even today.

As part of our planning, we've looked real hard at how we use our power and have decided on quite a few changes. First thing we're doing is getting rid of the blasted TV's and adding a Tuner card to my computer and using a 32 inch 1080 display for a monitor. I'm also including a Cintiq 13HD (1080 graphics tablet) as a secondary monitor ( we can use either for watching TV. Sound comes from a decent sound system for the house. Uses less power then even an HDTV does while the monitors are in sleep mode. Another option is the use of 12v Halogen lighting. That's a combination of track fixtures for task lighting and puck lights/cans for area lighting and they're fed 12v not 120 stepped down. We've even considered RV style 12v Flourescent fixtures but I like Incadecent lighting much more and standard bulbs work fine on 12v - just a bit less output though they do last longer. Lots of things we're still looking at but so far we've managed to cut our avg. power demand to less then 5Kwh (within our systems rated output) and all of that's just from thinking. It's even impacting our current demand since we're now cutting things off instead of leaving them on (no standy parasites) and printers are some of the worst offenders.

On our local parking lot solar panel "roofs", there was an inverter for every two "sets" of panels. I think there were 3 panels per set. Some installations use a microinverter per panel. Most of the roof top installations here use one inverter for 12-16 panels.

As I understand it, the more inverters there are, the less sensitivity there is to shadows; instead of losing a large group, you only lose one/a few panels. The other advantage of lots of inverters is the quantity of the DC cabling is minimized. All the long runs of wires are AC which greatly reduces the gauge needed and reduces the losses.

As I understand it, the more inverters there are, the less sensitivity there is to shadows; instead of losing a large group, you only lose one/a few panels. The other advantage of lots of inverters is the quantity of the DC cabling is minimized. All the long runs of wires are AC which greatly reduces the gauge needed and reduces the losses.

Neil, you almost have it correct. The inverter converts whatever DC you have to AC. Charge controllers optimize the output of the panels each charge controller is connected to. In somer cases there's 1 charge controller built into the inverter and it's still may only be referred to as an inverter, though in this configuration it's doing 2 functions. Individual charge controllers can handle any number of panels within their spec, however they will optimize for the panel with the lowest output, hence if 4 panels are on 1 controller and 1 panel is very shaded all 4 panels will have much lower output. The way around that of course is to have a charge controller for each panel, but in the past that's been a very expensive proposition. A single charge controller is fine for a small number of panels that don't have to deal with any shade besides clouds.

This is a very simple configuration. It's strictly a grid-tie because there's no battery backup, which really lowers up front costs, weight, bulk. You need a charge controller for each panel. They are not that high priced and they offer some unique advantages. They allow you to use panels of different sizes and capacities. The panels are all wired in series, to produce as much as 500 volts DC, which means a much smaller gauge wire can be used. It also means the inverter doesn't work as hard.

The safety advantage this system offers is tremendous. All grid tie systems require an on/off switch to be accessible in emergencies per code. It disconnects the inverter from the grid but most systems don't account for the DC that's still present. This is a bad situation for any fire fighters that have to deal with your home. With the Solar Edge setup each controller communicates with the inverter and only pass current when all the handshaking is complete. When you turn off the inverter all DC stops as well. I haven't seen any other setups that address this potentially deadly aspect of solar setups.

With less current to deal with and a simplified configuration these inverters are pretty reasonable priced. The only thing that drove me absolutely insane is when they show the circuit panel in the inverter. Here's a device that should last 30+ years, and like a cyclops there's this one large electrolytic capacitor sitting on the board. Really?

_________________People who put money and political ideology ahead of truth and ethics are neither﻿ patriots nor human beings.

As I understand it, the more inverters there are, the less sensitivity there is to shadows; instead of losing a large group, you only lose one/a few panels. The other advantage of lots of inverters is the quantity of the DC cabling is minimized. All the long runs of wires are AC which greatly reduces the gauge needed and reduces the losses.

Neil, you almost have it correct. The inverter converts whatever DC you have to AC. Charge controllers optimize the output of the Home solar system installation each charge controller is connected to. In somer cases there's 1 charge controller built into the inverter and it's still may only be referred to as an inverter, though in this configuration it's doing 2 functions. Individual charge controllers can handle any number of panels within their spec, however they will optimize for the panel with the lowest output, hence if 4 panels are on 1 controller and 1 panel is very shaded all 4 panels will have much lower output. The way around that of course is to have a charge controller for each panel, but in the past that's been a very expensive proposition. A single charge controller is fine for a small number of panels that don't have to deal with any shade besides clouds.

This is a very simple configuration. It's strictly a grid-tie because there's no battery backup, which really lowers up front costs, weight, bulk. You need a charge controller for each panel. They are not that high priced and they offer some unique advantages. They allow you to use panels of different sizes and capacities. The panels are all wired in series, to produce as much as 500 volts DC, which means a much smaller gauge wire can be used. It also means the inverter doesn't work as hard.

The safety advantage this system offers is tremendous. All grid tie systems require an on/off switch to be accessible in emergencies per code. It disconnects the inverter from the grid but most systems don't account for the DC that's still present. This is a bad situation for any fire fighters that have to deal with your home. With the Solar Edge setup each controller communicates with the inverter and only pass current when all the handshaking is complete. When you turn off the inverter all DC stops as well. I haven't seen any other setups that address this potentially deadly aspect of solar setups.

With less current to deal with and a simplified configuration these inverters are pretty reasonable priced. The only thing that drove me absolutely insane is when they show the circuit panel in the inverter. Here's a device that should last 30+ years, and like a cyclops there's this one large electrolytic capacitor sitting on the board. Really?

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